Electronically commutated, permanently-excited synchronous machines, also known as brushless motors, comprise a stator having at least two, in particular three, phase windings, and a rotor having at least one pole pair that is arranged perpendicular with respect to the axis of rotation, said pole pair being formed by means of one or multiple permanent magnets that are arranged in or on the rotor. A brushless motor is supplied with current by way of a control circuit, in which each phase winding is allocated an upper and a lower switch and said switches can influence each phase winding with an upper or a lower supply voltage. If one or multiple phase windings are energized, the rotor thus aligns itself in the magnetic field that occurs. It is necessary to determine the rotor position in order to control the brushless motor in an expedient manner, and said rotor position is determined by means of a resolver or rotary encoder.
Expediently, it is possible to perform a closed-loop control of the currents by means of the phase windings in a coordinate system that is fixed to the rotor, wherein a d-axis in the direction of the rotor magnetic field and a q-axis that stands at 90° (electrical angle, linked to the mechanical angle by way of the pole pair number) to this perpendicular d-axis are considered. A current that flows in the q-axis direction determines (in a motor without reluctance torque) the torque that is output and is therefore described as the torque-forming current (iq). In order to achieve high rotational speeds, it is possible to perform a field-weakening closed-loop control process in which a current that also flows in the d-axis direction is predetermined. The coordinate system that is fixed to the rotor rotates with respect to the stator, therefore the phase currents or corresponding phase voltages that are to be applied are determined by way of a suitable transformation with reference to the rotor position. It is also possible to predetermine phase voltages by way of alternative methods such as for example an open-loop control with the aid of values that are stored in a table. In accordance with the phase voltages that are determined, periodic control factors and also time durations that correspond to said control factors are determined in a pulse width modulation (PWM), such as in particular a space vector modulation, during which the respective phase windings are connected to the upper or the lower supply voltage by means of the control circuit.
Switching effects in the inverter of the control circuit are a relevant source for grid-bound interferences that can produce malfunctions in electronic circuits by way of the on board network. Both interferences in the range of switching frequency as well as their harmonic waves in the case of multiples of the switching frequency are generated by means of switching power semiconductors that control the phase windings. In terms of the electromagnetic compatibility (EMC), it is just as important in the case of electric motors as consumers with a relatively high electrical power to minimize the output of interference pulses. For this reason, a centered, multiphase PWM is frequently performed that produces a broader frequency spectrum with respect to a flank-centered PWM since the input and output switching flanks in the case of a rotating motor are continuously altered in relation to the fixed time pattern that is provided by means of the PWM base frequency. The associated frequency spectrum of the interference pulses that are output is consequently smoothed and the amplitude values of the switching frequency and harmonic waves that are determined are therefore smaller.